The identification procedure for identifying the position of a rotor in a sensorless brushless DC motor (BLDC), normally called “inductive sense,” takes advantage of the different response of the current in the motor winding with respect to a voltage pulse applied to the ends of the windings of the motor itself. The knowledge of the rotor position permits optimizing the motor startup procedure and is therefore a very important factor. The procedures of identifying the rotor position should therefore be characterized by high performances in terms of precision and insensitivity to disturbances.
Indeed an error in detecting the rotor position may result in an imprecise excitation sequence of the stator phases and in a consequent efficiency reduction or, in the worst case, in a failure of the motor start-up procedure (loss of synchronism).
The previous methods of detecting the rotor position (inductive sense) may only ensure good performance under conditions of unchanging supply voltage and precise current measurement.
The methods commonly used for detecting the position of the rotor, so called inductive sense methods, are based on the analysis of the current in the windings based upon a current pulse applied to the windings themselves. The different responses to the current pulses are based on the magnetic saturation level of the windings, which is a function of the position of the rotor position. In this way, the magnetic saturation level allows detection of the rotor position.
EP 1309078 discloses a method for detecting the position of a rotor of a DC motor with N phases having a plurality of windings. The method includes the steps of coupling at least two of the windings between the supply voltage and ground according to a first current path for a prefixed time (Tact), allowing the current stored in the two windings to discharge through a second current path, comparing the voltage at the ends of one of the two windings with a prefixed voltage and providing a control signal when the voltage is smaller in absolute value than the prefixed voltage. The method also includes performing the above steps for each of the windings of the DC motor and detecting the position of the rotor based on the control signals obtained.
This method is sensitive to the power supply variation because the current discharge is obtained by forcing the power stage at the high impedance condition, and the current charge is obtained by coupling the windings between the power supply and ground (current peak is function of the power supply).
US 2012/0098474 discloses an apparatus for detecting a position of a rotor of an electric motor having three phases and a plurality of windings. The apparatus includes circuitry to couple at least two of the windings between a supply voltage and ground according to a first current path, disconnect the at least two windings and allow the current stored in the two windings to be discharged through a second current path. The apparatus includes a measurement circuit adapted to measure the time period between the start instant of storing the current in the two windings and the final instant of discharging the current in the two windings, and a rotor detector for detecting the rotor position based at least in part on the measured time period.
Even this apparatus is sensitive to the power supply variation because the current discharge is obtained by forcing the power stage at the high impedance condition and the current charge is obtained by coupling the windings between the power supply and ground.
Also, apparatus for detecting the position of a rotor of a DC motor are known which use one sense element for each motor winding. The sense elements are embedded and coupled to the low side (or high side) transistors of the half bridges of the power stage. In this case, these apparatus are sensitive to the intrinsic mismatch of the embedded sense elements.